The present invention relates to a storage comparator operating by means of field-effect transistors formed as an integrated circuit principally on gallium arsenide and intended for digital transmission systems.
The digitalization of information and the processing thereof in real time is a technique which finds increasing application in present-day electronic systems. For this reason, the components have to handle larger and larger flows of digital information and the development of numerous systems seems at present conditioned by the availability of components capable of effecting the elementary logic operations in times appreciably less than a nanosecond. Gallium arsenide integrated circuits possess characteristics permitting them to bring an answer to this problem. The technology of negative threshold voltage gallium arsenide field-effect transistor integrated circuits (MESFET) enables the storage comparator of the invention to be constructed with a flow of 900 Megabits/second.
The development of communications entails the development of digital transmission systems whose information flow rates become larger and larger. When the information is to be conveyed over a long distance, the information which is necessarily degraded in the transmission channel must be regenerated. This is why there is disposed, at regular intervals, in the channel a means--the repeater--whose role is precisely to regenerate the electric or optical signal carrying the information. This repeater generally has a circuit which processes electric signals and which is called storage comparator or more rapidly regenerator and whose 3 functions are:
discrimination of the nature of the symbol present at the input to the generator at a precise time defined by the clock of the system;
resynchronization of the digital sequence with respect to the clock;
reshaping of the electric pulses.
In practice, this regenerating circuit operates like a type-D flip-flop having an adjustable voltage threshold for discriminating the nature of the symbol.
The storage comparator of the invention is intended for a high-flow (900 Megabits/second) digital transmission system on a coaxial cable. In such a system, the storage comparator forms the decision means for the regenerating repeater whose role it is to restore the signal degraded during transmission. The storage comparator of the invention processes electric signals.
The storage comparator is a binary component whose essential function is to determine at a so-called sampling time, imposed by an external clock, the electric signal received. In the system considered, digital information at 900 megabits/second is transmitted in the form of a signal with a very wide band, ten octaves or so, coded at three levels written symbolically:+1, 0, -1. The regenerating repeater comprises two storage comparators with staggered thresholds working in parallel, one of the storage comparators comparing the level of the signal received with respect to level +1, the other comparator with respect to the level -1.
Insofar as known storage comparators operating over a coaxial cable at flow rates greater than 90 Megabauds are concerned, the taking into account of the clock signal must be distinguished depending on whether the circuit triggers on a signal edge or whether the circuit functions with a very narrow pulse compared with the duration of the elementary pulse transmitted. This latter technique has been used for different existing systems but it presents the disadvantage of requiring a very narrow clock pulse of a duration less than 100 picoseconds for a flow rate greater than 600 Megabauds, which is not easily achievable especially when the power supply is reduced. It requires moreover the use of diodes such as tunnel diodes or step-recovery diodes whose consumption is fairly high and which do not have an excellent reputation for reliability.
Triggering on a clock pulse leading edge imposes for its part a greater complexity on the regenerator. Discrimination of the nature of the symbol is achieved either by using the non-linearity of the diode, or a differential circuit in structures derived from basic diagrams of the so-called ECL logic, or finally a Schmitt trigger. The active components used in these circuits are bipolar transistors and diodes on silicon or germanium for the earliest system. The highest flow rate known up to date is of the order of 560 Megabauds.
In a transmission system on a coaxial line, it is the regenerator and not the other elements of the repeater, such as filters or amplifiers, which limit to this value the on-line flow rate.
Known circuits formed by bipolar technology on silicon are then at present limited to an on-line flow rate of about 600 Megabauds. To operate with higher flow rates, the circuits formed on gallium arsenide are perfectly adapted since their operating speed is up to six times greater than the operating speed of circuits formed on silicon. This value being the theoretical value which corresponds to the ratio of the mobilities of the electrons in the silicon and in the gallium arsenide.